1. Field of the Invention
The invention relates to a method of scanning a radiation-sensitive recording surface of a rotating disc-shaped carrier along pedetermined concentric or spiral paths by means of a modulated radiation beam which is aimed at a target point on the record carrier, the target point being moved over the recording surface in a radial direction in accordance with a predetermined motion by positioning and beam deflecting means of a first.
The invention also relates to optical write apparatus for carrying out such method, comprising a drive means for rotating a disc-shaped carrier having a radiation sensitive recording surface, an optical system for aiming a radiation beam at a target point on the recording surface, and a positioning device for moving the target point across the recording surface in a radial direction within a first displacement range under control of a control device.
2. Description of the Related Art
Such a method and apparatus are known from Netherlands Patent Application No. 72 12 045 and the corresponding U.S. Pat. No. 3,894,179, issued July 8, 1975. Said Application patent describes a method and apparatus for the manufacture of master discs for making copies of optically readable record carriers. In accordance with said method a disc-shaped carrier having a recording surface formed by a photoresist layer is rotated by a drive means. As the carrier is rotated it is scanned by a laser beam which is aimed at a spot on the photoresist layer by the optical system. The drive means is arranged on a slide which moves it in such a way that the target point of the laser beam on the photoresist is radially moved over a displacement range of the order of magnitude of the radius of the disc-shaped carrier. As a result of the rotation of the carrier and the continuous radial displacement of the landing point of the laser beam the photoresist layer is scanned along a spiral path, the laser beam being digitally modulated in accordance with an on-off pattern determined by the information to be recorded. After the photoresist has been developed and the carrier has been subjected to a photo-etching process, a master disc with an information structure arranged along a spiral track is obtained which can be used for making copies.
The known method and apparatus have the disadvantage that they are less suitable for the manufacture of master discs for optically readable record carriers provided with a multiple spiral, for example record carriers formed with a spiral groove between which a spiral information track is arranged. By means of the known method and apparatus a disc having such a multiple spiral can be obtained only by first scanning the recording surface along one of the sub-spirals (for example the sub-spiral defined by the groove) and subsequently scanning the recording surface along the other sub-sprial defined by the information track, which requires a reversal of the radial direction of movement. As a result of hysteresis, which is caused by backlash and the like resulting from the direction reversal in the positioning device an accurate positioning of the starting point of each scan of the second sub-spiral between the turns of the first sub-spiral already scanned is not possible. This presents a serious problem, in particlar, in the manufacture of masters, where extemely stringent requirements are imposed on the positional accuracy (.+-.20.10.sup.-9 m) of the second sub-spiral relative to the first sub-spiral.
Moreover, in the manufacture of master discs for record carriers having a structure arranged along concentric tracks the known method and apparatus have the drawback that during radial positioning undesired vibrations arise as a result of the inevitable changes in slide velocity which occur when changing over from one concentric path to another. These undesired vibrations make it necessary to allow some extra waiting time before changing to another concentric path to allow the vibrations to be damped out sufficiently before recording can be resumed.